go/types: slice-to-array-pointer conversion requires go1.17

This is a port of CL 315169 to go/types. It uses a slightly different
mechanism for evaluating the convertibility error message, to be
consistent with operand.assignableTo.

Change-Id: Iea2e2a9fbb4cf17d472b2b7392786118e079528a
Reviewed-on: https://go-review.googlesource.com/c/go/+/315809


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>